1. Field of the Invention
The present invention relates to organic crystals and crystalline films, and more particularly to organic monocrystalline films.
2. Description of Related Art
Electro-optic devices, such as light modulators and shutters, have been widely applied in many areas. However, the devices that are available on the market are made from inorganic crystals. Generally speaking, these devices are not only relatively expensive and bulky, but also have a slow response time and need higher driving voltages.
Organic electro-optic materials based on extended xcfx80-electron systems have been proposed for electro-optic applications, especially in the area of fast opto-electronics, because of their large electro-optic coefficients and fast response time. For example, the electro-optic coefficient r11 of 2-methyl-4-nitroaniline (MNA) is about twice as large as the electro-optic coefficient r33 of LiNbO3, and the response time of MNA is expected to be 100 times faster than that of LiNbO3 at 633 nm (Lipscome et al., J. Chem. Phys 74:1509, 1981; Garito et al. Laser Focus 18:59, 1982). Other crystals, such as styrylpyridinium cyanine dye (SPCD), 4-N,N-dimethylamino-4xe2x80x2-Nxe2x80x2methyl-stilbazolium tosylate (DAST), and N-(4-nitrophenyl)-L-prolinol (NPP) have been reported to have very large electro-optic coefficients. However, a good organic crystal, in addition to having a fast response time and a large electro-optic coefficient, should also possess adequate chemical, thermal, and mechanical stability.
Generally speaking, organic electro-optic materials are available in the form of small flakes, which have the reputation of being fragile when subjected to mechanical, thermal, or chemical attack. Moreover, molecular crystals are fragile, since the interactions among the molecules are very weak.
Films of organic crystalline materials can be used in various optical applications. For example, optical waveguides made from organic material include slab waveguides in which light is confined in only one dimension, and 2-D channels of size 1 cmxc3x971 xcexcmxc3x971 xcexcm. In addition, cylindrical geometry devices filled inside and outside by a non-linear material and surrounded by an electrical field have been reported.
The growth of thin single crystals is discussed, for example, in the reference by K. M. M. Kruse entitled xe2x80x9cApparatus and Method For The Growing Of Single Crystal Specimens Of Organic Substances For Infrared Spectroscopic Investigation,xe2x80x9d J. of Physics E: Scientific Instr., vol. 3, pp. 609-14, 1970, Great Britain. The crystals are grown from the melt between NaCl (or KBr) windows held at a distance of about 25 xcexcm apart by means of a thin spacer (Polyester foil). A temperature gradient is maintained first along a capillary and then along the largest dimension on the NaCl windows, resulting in a clear definition of the crystallizing zone. The crystallizing zone is slowly raised by lowering the cell along the vertical temperature gradient within a heated column.
A second reference by Pech et al. studies the growth of solid benzophenone from its own melt contained in a crucible submitted to a unidirectional temperature gradient. (Pech et al., xe2x80x9cA New Technique For Determining The Kinetics Of Crystal Growth From The Melt,xe2x80x9d J. of Crystal Growth, vol. 43, no. 1, 123-25, 1978.) In this reference, a sample was placed in a 1xc3x9710xc3x9715 mm3 crucible made of glass plates. Thermal boundary conditions were imposed by two heating blocks fixed to the extremities of the crucible and connected to thermostats. The crucible is fixed to the substage of a microscope so that the moving liquid-solid interface can be maintained in coincidence with the cross-hair reticule of the objective of the microscope.
Another method for producing a crystal film is discussed in U.S. Pat. No. 5,385,116 to Hattori et al., entitled xe2x80x9cMethod For Producing Organic Crystal Filmxe2x80x9d. In this patent, a crystal film on an organic compound is produced from a molten liquid between a pair of substrates. At least one of the pair of substrates has on a part of a surface thereof a three-dimensional geometrical structure capable of controlling the direction of crystal growth of the organic compound. The other part of the surface having the three-dimensional geometrical structure is smooth.
U.S. Pat. No. 4,793,893 to Thakur et al., entitled xe2x80x9cMethods For The Preparation Of Thin Large-Area Single Crystals Of Diacetylenes And Polydiacetylenesxe2x80x9d, discusses a method for preparing thin large-area single crystals of diacetylene monomer. This method involves forming a liquid layer containing pure diacetylene monomer between two opposed surfaces; applying pressure to the liquid layer disposed between the two opposed surfaces; and crystallizing the liquid layer disposed between the two opposed surfaces while, by evaporation, the liquid layer is kept under constant pressure to form a thin large-area single crystal of pure diacetylene monomer. This patent also discusses a method for preparing a thin large-area single crystal of pure diacetylene monomer.
Notwithstanding the above mentioned references, there continues to exist a need in the art for improved structures including organic crystals and related methods for forming organic crystals. Such structures can provide organic crystals and crystalline films with superior optical properties.
In view of the foregoing background, it is an object of the present invention to provide improved methods and structures for organic crystalline films. These methods and structures can be used to create organic crystals and organic crystalline films with superior optical properties that can be used in electro-optical devices. The organic crystals according to the present invention have faster response times and use smaller driving voltages than currently-available inorganic crystals. These crystals are also mechanically, thermally, and/or chemically stronger than currently available organic materials.
This and other objects, features, and advantages according to the present invention are achieved by providing a device for manufacturing organic crystal films that protects the fragile crystals. The device can also be used directly as an electro-optical device and can comprise a first plate having a first face. The first face defines a recess which is filled with a material which can be crystallized, and covered with a second plate having a second face. Accordingly, the second face is in contact with the first face and the material in the recess is completely enclosed by the first and second plates. The material within the recess is thereby protected by the plates from chemical and mechanical damage as well as evaporation.
The material in the recess can be crystallized, and the step of crystallization can include the steps of heating the material above a melting temperature, and cooling the material to obtain a homogeneous distribution. The crystallization step can be used to produce a single crystal film of the material in the recess. The single crystal film can provide optical properties that are superior to those of polycrystalline films. In addition, the material can be an organic compound which has non-linear optical properties.
The plates used to contain the material are preferably transparent allowing the material to be visually monitored during the crystallization step. For example, the plates can be formed of fused quartz, which is preferably optically polished to reduce the formation of defects during the crystallization step.
Furthermore, the first face of the first plate preferably defines a groove surrounding the recess. This groove can then be used to contain a portion of the material. For example, if the recess is initially overfilled, or if the material overflows from the recess as a result of thermal expansion, the excess can be contained by the groove. Accordingly, any excess material will be prevented from separating the two plates.
In another embodiment of the present invention, an optical device includes a first plate having a first face which defines a recess, a crystalline material in the recess, and a second plate having a second face wherein the second face covers the first face and crystalline material in the recess. The first face of the first plate may further define a groove surrounding the recess, and a portion of the crystalline material may be contained in the groove surrounding the recess.
The first and second plates are preferably first and second transparent plates such as first and second fused quartz plates. In addition, the second face of the second plate and a surface of the recess opposite the second plate are preferably optically polished. The crystalline material may be an organic compound, and it is preferably a single crystal film.
The methods and structures of the present invention provide an organic single crystal thin film which can be used in non-linear optical applications. The structure can be reproducibly fabricated with relatively little expense. In addition, the thin film in the recess is protected from chemical and mechanical damage, as well as evaporation, by the two plates.